1. Technical Field
This invention relates generally to memory devices, and more particularly, to a write-once read-many times memory.
2. Background Art
A write-once read-many times memory is a storage medium to which data can be written only a single time, but can be read a large number of times. Such a storage medium provides substantially longer “shelf life” than a magnetic storage medium, and thus is highly useful when data must be preserved for a long time. Typically, such a memory takes the form of an optical disk, for example, a compact disc on which data is permanently etched during the write process. In addition to long data retention, is desirable that the memory provides high density and low power usage, accompanied by fast write and read speeds.
While this form of write-once read-many times memory has proven advantageous, it will be understood that improvements in data density, power usage, and write and read speeds is constantly being sought.
FIG. 1 illustrates a programmable and erasable memory device 30. The memory device 30 includes a Cu electrode 32, a Cu2S passive layer 34 on the electrode 32, a Cu2O active layer 36 on the layer 34, and a Ti electrode 38 on the active layer 36. Initially, assuming that the memory device 30 is unprogrammed, in order to program the memory device 30, an increasingly negative voltage is applied to the electrode 38, while the electrode 32 is held at ground, so that an increasing electrical potential is applied across the memory device 30 from a higher to a lower potential in the direction from electrode 32 to electrode 38, until electrical potential Vpg (the electrical potential required to program the memory device 30) is reached (see FIG. 2, a plot of memory device current vs. electrical potential applied across the memory device 30). This potential Vpg is sufficient to cause copper ions to be attracted from the superionic layer 34 toward the electrode 38 and into the active layer 36, causing the active layer 36 (and the overall memory device 30) to switch to a low-resistance or conductive state (A). Upon removal of such potential (B), the copper ions drawn into the active layer 36 during the programming step remain therein, so that the active layer 36 (and memory device 30) remain in a conductive or low-resistance state, as indicated by the resistance characteristic (B).
In order to erase the prior art memory device (FIG. 2), an increasingly positive voltage is applied to the electrode 38, while the electrode 32 is held at ground, so that an increasing electrical potential is applied until electrical potential Ver (the “erase” electrical potential) is applied across the memory device 30 from a higher to a lower electrical potential in the reverse direction. With bonding between the copper ions and the active layer not being strong, this potential Ver is sufficient to cause copper ions to be repelled from the active layer 36 toward the electrode 32 and into the superionic layer 34 (C), in turn causing the active layer 36 (and the overall memory device 30) to be in a high-resistance or substantially non-conductive state. This state remains upon removal of such potential from the memory device 30.
FIG. 2 also illustrates the read step of the memory device 30 in its programmed (conductive) state and in its erased (nonconductive) state. An electrical potential Vr (the “read” electrical potential) is applied across the memory device 30 from a higher to a lower electrical potential in the same direction as the electrical potential Vpg. This electrical potential is less than the electrical potential Vpg applied across the memory device 30 for programming (see above). In this situation, if the memory device 30 is programmed, the memory device 30 will readily conduct current (level L1), indicating that the memory device 30 is in its programmed state. If the memory device 30 is erased, the memory device 30 will not conduct current (level L2), indicating that the memory device 30 is in its erased state.
Reference is made to the paper THEORY OF COPPER VACANCY IN CUPROUS OXIDE by A. F. Wright and J. S. Nelson, Journal of Applied Physics, Volume 92, Number 10, pages 5849-5851, Nov. 15, 2002, which is hereby incorporated by reference. That paper describes the process of diffusion of copper ions through Cu2O. In the diffusion process, typically involving a vacancy mechanism wherein atoms jump from a first (atom) state to a second (vacancy) state, atoms need energy to break bonds with neighbors and to provide necessary distortion of the material between the states. The above-cited paper indicates that the activation energy Ea for moving a copper ion from one state to the next in the Cu2O is approximately 0.3 eV. FIG. 3, a graph illustrating this movement, shows movement (arrow F) from state 1 (unprogrammed) to state 2 (programmed). In such process, the activation energy is indicated by the arrow Ea1. FIG. 4 illustrates movement (arrow G) from state 2 (programmed) to state 1 (unprogrammed). In such process, the activation energy is indicated by the arrow E2a. With the relatively low activation energy Ea2 required to move copper ions and from state 2(programmed) to state 1 (unprogrammed), programmed data may be easily lost.
This type of memory device provides many advantages, in particular, very high density, low power requirements, and very fast read and write capabilities. However, the memory device 30 as described, being readily erased, is not intended to meet the specification of a write-once read-many times memory.
What is needed a memory device which includes the advantages of the memory device 30 described above, i.e., high density, low power usage, and high read and write speeds, meanwhile being a write-once read-many times memory.